Postage meters, as well as other metering machines, typically include a stationary base and a rotatable print head. Information or instructions must be transferred from the stationary base to the moveable print head to position movable printing elements carried by the print head. The printing elements are typically in the form of print wheels having indicia about their peripheries. Each print wheel is rotated until the chosen indicium for that print element is properly positioned. The properly positioned print wheels provide the desired impression, such as the value of postage on a letter, when the print head is cycled one revolution.
Although many different ways have been devised to transmit information from a stationary element to a movable element, in some situations, such as in dealing with postage meters, security requirements limit the options. Specifically, with postage meters one needs a positive, anti-fraud linkage between the print head and the base. This is best achieved using a mechanical coupling between the elements together with anti-fraud elements. The anti-fraud elements are intended to prevent the movement of the printing element once the print head is moved from its home position unless the user damages the meter.
The mechanical linkage used in a conventional postage meter is shown in U.S. Pat. No. 4,658.122 issued on Apr. 14. 1987, the disclosure of which is incorporated by reference. That patent discloses an electro-mechanical meter in which the value to be printed is entered through a keyboard which causes a number of rods axially. The ends of the value rods extend into the print head and are connected to racks which engage gear teeth on the print wheels. The value rods rotate with the print head to provide the positive mechanical coupling between the base and the print head. Various types of anti-fraud measures intended to keep the unscrupulous user from rotating the print wheels after the print head has been moved from its home position have been devised. One is shown in U.S. Pat. No. 4,777,353 dated Oct. 11. 1988.
Although the mechanical couplings and interlocks devised to date do a very good job of securely transferring information from the base to the print head, there remains room for improvement on the basis of mechanical simplicity and anti-fraud integrity.